The tibial insert, usually of polyethylene, is mounted so as to be freely rotatable relative to the tibial plate (generally of metal) so as to permit clearance in proper rotation of the femur relative to the tibia, the proper rotation occurring via the condyles (usually of metal) which slide on the corresponding sliding surfaces of the tibial insert. In the unbent position (knee bending angle .alpha.=0.degree.) of the knee, rotation of the femur relative to the tibia is blocked by the condyles abutting on the polyethylene insert. As soon as the knee bends, however, (.alpha. becomes greater than 0) the block is completely relaxed and the femur can rotate over a very wide angle before the knee has appreciably bent. This is not a true reproduction of the anatomical knee and may result in undesirable jamming.